1. Field of the Invention
The present invention generally relates to regulator circuits, and particularly relates to a regulator circuit used in an IC (integrated-circuit) card.
2. Description of the Related Art
An IC card is used in various industrial fields where information and data inside the card need to be exchanged with external devices, and is exemplified by a commuter pass, a basic resident register, a credit card, etc. A system for exchanging data with external devices includes a system using noncontact-antenna-based communication and a system using contact-node-based communication. A system for supplying power to drive circuitry inside the card includes a system based on a noncontact antenna and a system based on contact nodes.
In order to protect the security of information inside IC cards, functional improvements such as the provision of protection functions have been made, which necessitates a built-in CPU or the like in the IC card in addition to memories. Moreover, the fields of application have expanded, resulting in an increase of circuit size. As the circuit size increases inside the IC cards, power fluctuation caused by circuit operations becomes large, and required electric power also increases.
Accordingly, there is a need for a power supply that is capable of supplying stable, sufficient electric power even when a sudden change in the load occurs. FIG. 1 is block diagram showing an example of the construction of a related-art power supply circuit.
The power supply circuit of FIG. 1 includes an antenna 10, a rectifier 11, a digital volume 12, a series regulator 13, a capacitor C, and Zener diodes ZD1 and ZD2.
Electric power received at the antenna 10 is boosted by the operation of resonance effected by the inductance L of the antenna and the capacitor C connected in parallel, and is then supplied to the rectifier 11 as alternating voltage. The magnitude of resonance may become infinite if there is no load. The Zener diodes ZD1 and ZD2 are thus provided for the purpose of preventing excessive voltage.
The rectifier 11 converts the alternating voltage supplied from the antenna 10 into a direct-current voltage. The digital volume 12 serves as a pseudo load to adjust the input voltage VDP to a proper voltage level. The series regulator 13 attends to voltage control responsive to a load change, thereby supplying a constant direct-current voltage to the load.
In the case of making a contact, the antenna is not used, and power is supplied through terminals N1 and N2.
FIG. 2 is a circuit diagram showing a detailed circuit construction of the digital volume 12 and the series regulator 13 of FIG. 1.
In FIG. 2, the digital volume 12 includes resistors 21 and 22, an AD converter 23, transistors 24-0 through 24-m, resistors 25-0 through 25-m having respective resistances R0 through R0/2m, and a transistor 26. The series regulator 13 includes an operational amplifier 31, a transistor 32, resistors 33 and 34, and a transistor 35.
The resistors 21 and 22 divide a direct-current voltage VDP supplied from the rectifier 11 of FIG. 1. The AD converter 23 converts the divided potential into a digital value comprised of D0 through Dm, and controls the ON/OFF of the transistors 24-0 through 24-m according to this digital value. With this provision, the resistors 25-0 through 25-m having respective resistances R0, R0/2, . . . , and R0/(2m) are selectively coupled to the voltage VDP according to the digital value. The adjustment of combined resistance is such that the higher the detected value of the direct-current voltage VDP, the lower the combined resistance is. This achieves control that maintains the voltage VDP at a constant level. The series regulator 13 generates a voltage VDDF lower than the voltage VDP in the manner as follows. The voltage VDDF is divided by the resistors 33 and 34, and the operational amplifier 31 compares the divided potential with a reference potential. The operational amplifier 31 controls the gate voltage to make the transistor 32 more conductive if the divided potential is lower than the reference potential, and controls the gate voltage to make the transistor 32 less conductive if the divided potential is higher than the reference potential. Such feedback control by the operational amplifier 31 makes the voltage VDDF constant.
A control signal, when its voltage level is low, turns off the transistor 35, thereby suspending the control function of the operational amplifier 31. When the voltage level of the control signal is high, the control function of the operational amplifier 31 is turned on.    [Patent Document 1]    Patent Application Publication No. 10-240889    [Patent Document 2]    Patent Application Publication No. 2000-348152    [Patent Document 3]    Patent Application Publication No. 2002-288615    [Patent Document 4]    Patent Application Publication No. 2002-99887
The related-art regulator circuit shown in FIG. 1 and FIG. 2 has drawbacks as follows.
In the regulator circuit shown in FIG. 1 and FIG. 2, the transistor 32, which is controlled by the operational amplifier 31 of the series regulator 13, is connected in series to the load. If an increase in the load causes the output voltage VDDF to drop, the transistor 32 is controlled to be more conductive, thereby increasing a supply current. If a decrease in the load causes the output voltage VDDF to rise, the transistor 32 is controlled to be less conductive, thereby decreasing a supply current. In this construction, the electric current flowing through the transistor 32 is controlled so as to change the electric current running through the load, thereby controlling the voltage. If the response speed of the operational amplifier 31 is increased, a difference of speed between voltage changes and feedback control becomes large. There may thus be a risk of letting the operational amplifier 31 oscillate. Because of this reason, it is difficult to cope with a sudden change in the load by enhancing the response speed of the operational amplifier 31.
Moreover, during the operation of the regulator circuit of FIG. 1 and FIG. 2, the digital value of the digital volume 12 is set as a given proper value. Electric wave received at the antenna 10 not only includes an electric wave for power-supply purposes, but also includes superimposed AM modulated signals for the purpose of communicating information necessary for the IC card. Therefore, the response speed of the digital volume 12 for keeping the voltage VDP constant is set to a sufficiently slow speed that would not absorb the modulated signals.
If the load connected to the voltage VDDF shows a sudden change, the control of the voltage VDDF by the operational amplifier 31 may not be sufficiently fast, resulting in the voltage VDP on the input side being changed. Since the response speed of the digital volume 12 is slow as described above, the voltage VDP is let drop, failing to maintain the constant voltage level.
Accordingly, there is a need for a power supply circuit that supplies stable power by keeping the input voltage VDP and the output voltage VDDF constant against a sudden change in the load.